Parylene is a generic term often used to describe a class of poly-p-xylenes which are derived from a dimer of the structure: ##STR1## wherein X is typically hydrogen or a halogen.
Due to its ability to provide thin films and conform to substrates of varied geometric shapes, parylene is ideally suited for use as a conformal coating in a wide variety of fields, such as, for example, in the electronics, automotive, and medical industries. Parylene coatings are inert, transparent and have excellent barrier properties. When X in the above formula is hydrogen, parylene is particularly useful at temperatures of up to about 130.degree. C.
Octafluoro-[2,2]paracyclophane ("AF4") is a fluorine substituted version of the above dimer and has the structure; ##STR2## Parylene which is made from AF4 can also provide an inert, transparent conformal coating which has excellent barrier properties. In addition, parylene made from AF4 can be used at temperatures up to about 240.degree. C.
Accordingly, parylene made from AF4 is particularly useful for coating products that will be exposed to high temperatures, e.g., electrical components in automotive applications. However, despite the commercial need, AF4 has not been extensively used as a dimer for the production of parylene because the typical preparation method for AF4 involves pyrolyzing a monomer at temperatures in the range of about 600 to 1000.degree. C. These high temperatures have made the process uneconomical. The following patents describe the preparation of AF4 under pyrolysis conditions.
U.S. Pat. No. 3,268,599, issued to Chow, relates to a process for the preparation of cyclic alpha-perfluoro-di-p-xylylenes, specifically AF4, by the pyrolysis of a dihalide monomer. At column 1, line 51 to column 2, line 6, the patentee discloses;
Now in accordance with the present invention, cyclic .alpha.-perfluoro-di-p-xylylene having the structural formula ##STR3## is produced by the process which comprises forming a reactive intermediary p-xylylene diradical having the basic structure ##STR4## by the pyrolysis at temperatures between about 700.degree. C.-1000.degree. C. of compounds having the general formula ##STR5## wherein Y is a halogen having a lower bond strength than fluorine such as bromine, chlorine, and iodine, and Y' is a member selected from the group consisting of hydrogen and a halogen having a lower bond strength than fluorine and n is an integer from 1 to 2 inclusive, and cooling and condensing the thus formed diradicals in intimate admixture with a fluid medium containing an inert organic solvent maintained at a temperature above about 50.degree. C.
U.S. Pat. No. 3,274,267, issued to Chow, also relates to the preparation of AF4. However, the compounds disclosed in U.S. Pat. No. 3,274,267 have an aromatic nuclear substituent group on each of the benzene rings.
U.S. Pat. No. 3,297,591, issued to Chow, relates to a process for the preparation of parylene made from AF4 and contains a similar disclosure to U.S. Pat. No. 3,268,599 with regard to the preparation of AF4 by pyrolysis.
Chow, Pilato and Wheelwright in an article captioned The Synthesis of 1,1,2,2,9,9,10,10,-Octafluoro [2,2]paracyclophane, The Journal of Organic Chemistry, Volume 35, No. 1, 20-22, Jan., 1970, disclose a method for the preparation of AF4 which is similar to the method described in the above patents.
In order to allow for more widespread use of parylene made from AF4, it is desirable to produce AF4 by processes which can operate at low temperatures, i.e., non-pyrolysis conditions.